net: apps: Set the IPv6 source address preferred for echo-client
net: apps: Set the IPv6 source address preferred for echo-client

Forcefully set the IPv6 address in echo-client so that we
can use that address when sending data. Normally this should
not be done like this as the system should autoconfigure itself.

Change-Id: Id24f9aa84b30a3da941bb6b9e4522c7218687e12
Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

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